This circuit was obtained from Elektor magazine and published in the Encyclopedia of Electronic Circuits Vol 5 of 1995. Transistors are RF types that are still common today.
In the figure we have a symmetrical power supply stabilized by a zener diode. Transformer and...
This filter that can use any operational was found in a French documentation from 1982. The power...
This receiver for listening on the phone was found in a Japanese magazine in 1967. The circuit uses...